This invention relates to stabilizer compositions for halogen-containing vinyl polymers, the stabilized halogen-containing vinyl polymer compositions and articles formed therefrom, and methods for stabilizing halogen-containing vinyl polymers.
Halogen-containing vinyl polymers, for example, poly(vinyl chloride) (PVC), copolymers of vinyl chloride and vinyl acetate, and poly(vinylidene chloride), are commonly used for fabricating a variety of articles such as pipes, window casings, siding, bottles, wall covering and packaging film. There have been many attempts to add stabilizers to halogen-containing vinyl polymers to improve the color hold in the polymer resins, particularly the color hold within the processing window, e.g., during the initial processing and during any subsequent re-processing. Most stabilizer compositions in present use contain metals such as tin, cadmium, and even lead. While these stabilizer compositions can be effective for minimizing discoloration during initial processing, as well as during any subsequent re-processing, there has been increasing interest in developing stabilizer compositions that are free of tin, cadmium, and/or lead for environmental, cost, and other reasons.
Latent or blocked mercaptans have been found to be particularly effective for minimizing discoloration early in processing, as well as for providing long-term color stability, i.e., when the polymer is exposed to ultraviolet light and weathering. An advantage of the latent mercaptans is that release of the free mercaptan during processing can augment the effects of other stabilizers (i.e., such as organic or metallic stabilizers). An example of such combination is disclosed in U.S. Pat. No. 4,973,619, in which a latent mercaptan is combined with a fatty acid salt of cadmium or zinc.
Dihydropyridines may also be used as stabilizers for polyvinyl chloride polymer compositions as described in U.S. Pat. No. 4,209,439. It was shown that dihydropyridines are effective thermal stabilizers at lower concentrations than those used for 2-phenyl indole, a well-known thermal stabilizer. A combination of a dihydropyridines with an amino uracil has been described in U.S. Pat. Nos. 5,925,696 and 6,194,494.
Beta-diketones, another class of thermal stabilizers, may be used alone or in combination with other stabilizers. U.S. Pat. No. 3,001,970 describes the stabilization of vinylidene chloride with an ortho-hydroxy-dibenzoylmethane. U.S. Pat. No. 4,427,816 describes a synergistic combination of a beta-diketone and a hydrotalcite. In U.S. Pat. No. 5,015,777, a stabilizer composition comprising a beta-diketone, mannitol, and a mixture of magnesium and zinc salts of benzoic and fatty aliphatic acids is described.
While these prior art combinations are suitable for their intended purpose, there remains a need in the art for improved stabilizer compositions for halogen-containing vinyl polymers, particularly compositions that are free from cadmium, tin, and/or lead, and that provide improved resistance to discoloration during processing and/or use.